This invention relates to improvements in prestressed concrete pressure vessels (PCPV) subjected to internal pressures much higher than those normally encountered by any current PCPV, and to a method for forming the vessel. Pressures in the improved PCPV of this invention may be as high as 12,000 psi or even higher.
The PCPV of this invention may for example be used for petro-chemical and coal conversion plants. Such vessels may include hydrogen storage vessels, methanol exchange and converter vessels, and combined dissolver-separator vessels (DSV) used in coal liquefaction processes. Other applications requiring the containment of extremely high pressures are well suited for the new PCPV described herein, and thick-walled very costly steel vessels that have been required in the past can be replaced by the improved PCPV.
Since the innovative and pioneering days of PCPVs, the use of PCPVs has become established practice within the framework of codes and standards governing their use. The codes governing PCPVs serve reactor vessels representing conditions very different from those to which the present invention is directed. The governing code of practice has been the ASME Boiler and Pressure Vessel Code, Section III. Because of the nature of the vessels' application in most cases, the code on such pressure vessels is rightfully ultra-conservative. This means essentially that no tension is allowed in the concrete in the vessel wall. The concrete is required always to be in a state of compression induced thereon by circumferential prestressing in the external region of the vessel. While this requirement works well for types of pressure vessels which are subjected to internal pressures of the order of 1,000 psi, it practically puts a stop to using prestressed concrete as pressure vessel material in applications where internal pressures are much higher than 1,000 psi. E.g., coal conversion pressure vessels can be subjected to internal pressures up to 4,000 psi. Other applications, principally as test vessels, require the containment of pressures over 10,000 psi. According to conventional practice, increasing internal pressures are handled by increasing circumferential prestressing, or increasing wall thickness, or both. Limitations on how much external prestressing can be increased to cope with increasing internal pressure are imposed by the high tangential compressive stress generated by the external prestressing at the internal cavity surfaces. Also, limitations on vessel wall thickness are imposed by economic and practical considerations. These serious limitations prevent this method from solving the problems of containing high internal pressures, and a new method is therefore required.
Under conventional design practice for prestressed concrete pressure vessels, governed by codes as discussed above, stress limits for concrete under Service Levels A and B (Normal/Abnormal etc.) are zero for tensile stress, and 0.3 C f.sub.cua for compressive stress, where C is a stress enhancement factor due to the confinement of concrete that can vary to about 2.7 depending on the ratio of compressive stresses in the other two directions. These criteria ensure that the vessel concrete will remain predominantly in compression under operating conditions.
For a vessel similar to the DSV, if the external pressure exerted on the vessel by prestressing is held constant, an increase in the internal operating pressure would have to be matched by an increase in the wall thickness at a much faster rate, if tensile stress at the external region were to be held at zero as these conventional criteria demand. E.g., a 50% increase in internal pressure requires an increase of wall thickness by about 350%.
A more efficient way to counter increasing internal pressures would be to increase circumferential prestressing. However, this solution has been quickly limited in applicability by the high tangential compressive stresses that would be generated at the internal region of the wall, and by the congestion of prestressing steel at the exterior face.
In copending application Ser. No. 4,742, now U.S. Pat. No. 4,265,066, there was presented a new approach to the design and construction of moderately high pressure PCPVs. That system, developed from basic considerations of serviceability, behavior and safety, accepts tension and tension cracks in the outer region of the PCPV, and does not come within the purview of present codes. It provides for incorporation of artificially-introduced, preformed vertical separations at pre-determined crack locations as a method of localizing cracking and controlling high tensile stresses generated by the combined effects of internal temperature and pressure. A vessel formed according to the principles of that disclosure accepts a limited degree of tension in the outer regions of the concrete wall, and cracking separation is limited essentially to the preformed slots. A study has shown that the PCPV so designed was, in the extreme case of the DSV, approximately 70% less costly than the 18 steel vessels of equivalent capacity it was intended to replace.
Serviceability requires that the structure respond to all functional requirements throughout its operating life. The behavior of the structure is concerned with the known properties and characteristics of the component materials, and with their performance as a whole. Safety considerations refer to the reliability and redundancy of a system that are commensurate with the purpose, importance, and failure consequences of the structure. The cracked PCPV design is not less safe than an uncracked PCPV designed according the conventional codes.
Tensile stresses in a high-pressure PCPV are practically unavoidable. A more realistic assessment of tensile stresses in the strucutre is not whether cracking should be permitted, but whether such cracking will diminish the serviceability, behavior and safety of the structure for which it is designed. For a high-pressure PCPV with wall thickness of 15 ft. or more, the occurence of vertical tensile cracks extending radially from the outer surface to a depth that is less than half the thickness of the sizeable wall, should not impair the integrity and functional adequacy of the vessel.
The concept of a cracked high-pressure PCPV was developed on these considerations. To ensure that the radial cracks do not penetrate into the wall to more than half the thickness, the allowable tensile stress in the concrete in elastic analysis has been arbitrarily set at 0.6 f.sub.cua, of the same order of magnitude as confined state stresses allowed by the existing code for vessels.
Although the PCPV of the copending application has been successful and accepted, there is now need for a PCPV capable of containing ultra-high pressures, often well over 10,000 psi. Such pressures are encountered in certain applications, for example ocean simulators, hydrogen storage vessels, methanol conversion vessels, crystal growing vessels, urethane reactor vessels and similar reactor vessels used in some petroleum and chemical processing. The high pressures in these applications are not necessarily accompanied by high internal temperatures. They are in this way different from the moderately high pressures, e.g. 2000-4000 psi, for which the PCPV of the copending application, with outer stress relieving slots, was primarily directed. In those applications, the combined and additive effects of high internal temperature and pressure make tension at and adjacent to the outer surface the critical factor. The prestressed concrete DSV, for example, having internal cavities for a dissolver reactor and for a flash drum separator, contains internal operating pressures and temperatures which may be over 2000 psi and 850.degree. F. For the previously disclosed PCPV with outer stress-relieving slots, the DSV is an ideal application. On the other hand, where there is high internal pressure not accompanied by high temperature, the critical stress region may be the internal region, due to excessive tangential compression. In this case the vessel may be beneficially formed according to the principles of the present invention.
The present invention provides a revolutionary new application of the above-described principles of crack acceptance and design in accordance with serviceability, behavior and safety, to the design and construction of an ultra-high pressure PCPV, wherein tangential compressive stress at the internal cavity wall is the critical factor, rather than tension stress at the outer surface. Internal compressive stress is critical because of the extra high post-tensioning force that must be applied at the outside surface to contain the very high internal pressure, often well over 10,000 psi. As will be explained below, the PCPV of the present invention provides for the containment of these extremely high pressures with relatively thin walls, and with a great deal less cost than required for steel vessels of comparable capacity.